1. Field of the Invention
This invention relates to a magnetic brush developing device for use in an electrophotographic reproducing apparatus.
2. Discussion of the Prior Art
In general, magnetic brush developing devices are known in which there are formed bristles of a magnetic developer at magnetic poles appearing on the outer peripheral surface of a rotatable sleeve made of nonmagnetic material, but having built-in fixed magnets. The bristles of the above-mentioned magnetic brush slidingly contact an electrostatic latent image formed on the surface of a photosensitive member located in the immediate vicinity of the aforesaid rotatable sleeve for purposes of developing the image. With this known device, the developer which forms magnetic brush bristles at the magnetic poles on the outer periphery of the sleeve is carried on the rotating sleeve into the development area, and then removed by means of a developer-scraping plate, the removed developer and fresh toner supplied in a given amount from a toner container then being mixed by means of agitating blades provided downstream of the scraper, thus supplying replenished developer for use in development of subsequent images.
With the above-mentioned device, however, movement of the developer with the rotating sleeve produces friction due to contact with a bristle-limiting plate, the scraping plate and agitating roller, etc. which are located close to the peripheral surface of the sleeve, so that the toner is melted by frictional heating, thus adhering to the carrier. This adherence causes fatigue in the developer with resulting premature deterioration in copy quality, and thus it becomes necessary to replace developer frequently.
For retarding the above-described deterioration of the developer, the size of the developer reservoir could be increased so as to have a great amount of developer contained therein. Mere increase in the capacity of the reservoir leads, however, to poor efficiency of developer circulation, i.e., there is an increase in "dead" stock (developer which is not used in actual practice), thus providing only a partial solution to the problem.
In addition, with the aforesaid device, the developer is stirred by means of agitating blades only in the direction at a right angle to the sleeve axis rather than in line with the axis and tends to be biased to one side of the sleeve with respect to the width thereof, so that there arises an uneven distribution of developer over the sleeve. Accordingly, this brings about extremely uneven distribution of toner density over a resulting image.
The present invention is directed to avoiding the aforesaid shortcomings.